This invention relates to fluid filters and their method of manufacture and, more particularly, to hermetically sealed filters such as automatic transmission fluid filters.
Automatic transmissions require a filter to remove harmful materials from the fluid being circulated through the transmission system. Various configurations of filters have been suggested for use in automatic transmission fluid circulating systems. One such filter is disclosed in U.S. Pat. No. 4,402,827 to Joseph. This filter is made from two generally rectangular cup-shaped pan members having flanged peripheries fastened together in opposed relation. A filter element is made in part from a long piece of resin-impregnated felt, folded over double. The edges of the filter element are captured between the fastened flange sections of the pan members.
Like most automatic transmission filters, the Joseph filter is designed to be placed in the transmission fluid sump area on the inlet side of the fluid pump. For this reason, it has an outlet tube extending up from the top pan member which conveys fluid to the pump. The inlet to the filter is a hole in the pan member positioned so as to be submerged in transmission fluid within the sump.
It is critical that the inlet to the filter remain submerged in transmission fluid. If air is allowed to pass through the filter, it could cause cavitation in the transmission pump and, with a loss in hydraulic pressure to the clutch slippage between the clutch plates. Slippage results in a burning of the drive surfaces of the clutch and complete transmission failure within seconds after slippage begins.
For filters like those in the Joseph patent, having relative wide, flat bodies, the edges of the filter become exposed to air during normal operation of the automobile in which they are used. During violent maneuvers in starting and turning, the fluid in the sump is subjected to forces causing it to slosh to one side or another. If the filter edges are not sealed, air can enter the filter and have the above-described effect.
In the past, filters made from a plastic top pan member and a metal base pan member have been assembled by crimping the edge of the metal around the edge of the plastic member. Tests have shown that such crimping is not effective for sealing the edges airtight. One of the major difficulties in getting such filter members sealed on the edges is that the filters are subject to operating conditions ranging from -30.degree. to +300.degree. F. Under these conditions, it is very difficult to maintain an airtight crimp seal with dissimilar materials. In addition, many sealant materials are not capable of withstanding this operating range.
In the first parent application, now U.S. Pat. No. 4,826,598, a hermetically sealed transmission filter is disclosed. The preferred embodiment of the filter is similar in shape to the filter of the Joseph patent, with a plastic cover member and a metal base member. The filter is provided with a hermetic seal by molding a plastic overmold onto the periphery of the filter after the cover and base members have been placed into alignment with the non-folded edges of the filtration media captured between flanges at the periphery of the filter.
In the second parent application, now U.S. Pat. No. 4,828,694, another hermetically sealed transmission filter is disclosed, again of the shape similar to that of the Joseph patent. Again, the filter is provided with a hermetic seal by applying an overmold to the periphery of the filter. One difference between the filters of the second parent application is that both housing members are made of thermoplastic material.
The disclosures of both U.S. Pat. Nos. 4,826,598 and 4,828,694 are incorporated herein by reference.
One of the difficulties with providing a plastic overmold to produce the required hermetic seal disclosed in the filters of the two parent applications is that the filters must have the overmold applied in an injection molding tool after the pieces are assembled. Not only does this require an additional mold, but it increases production time, since the pieces are generally molded separately, assembled and then accumulated for later overmolding.